Biology
& Chemistry

Anaerobic
Fermentation

Composting
without oxygen results in fermentation. This causes organic compounds
to break down by the action of living anaerobic organisms. As in
the aerobic process, these organisms use nitrogen, phosphorus,
and other nutrients in developing cell protoplasm. However, unlike
aerobic decomposition, this reduces organic nitrogen to organic
acids and ammonia. Carbon from organic compounds, is released mainly
as methane gas (CH4). A small portion of carbon may be respired
as CO2.

This
anaerobic process takes place in nature. Examples include decomposing
organic mud at the bottom of marshes and buried organic materials
with no access to oxygen. Marsh gas is largely methane. Intensive
reduction of organic matter by putrefaction is usually accompanied
by unpleasant odors of hydrogen sulfide and of reduced organic
compounds that contain sulfur, such as mercaptans (any sulfur-containing
organic compound).

Since
anaerobic destruction of organic matter is a reduction process,
the final product, humus, is subject to some aerobic oxidation.
This oxidation is minor, takes place rapidly, and is of no consequence
in the utilization of the material.

There
is enough heat energy liberated in the process to raise the temperature
of the putrefying material. In the anaerobic dissolution of the
glucose molecule, only about 26 kcal of potential energy per gram
of glucose molecules is released compared to 484 to 674 kcal for
aerobic decomposition. The energy of the carbon is in the released
methane (CH4). The conversion of CH4 to CO2 produces large amounts
of heat. This energy from anaerobic decomposition of organic matter
can be used in engines for power and burned for heat.

Pathogens
could cause problems in anaerobic composting because there is not
enough heat to destroy them. However, aerobic composting does create
high enough temperatures.
Although heat does not play a part in the destruction of pathogenic
organisms in anaerobic composting, they do disappear
in the organic mass because of the unfavorable environment and
biological antagonisms. They disappear slowly. The composted material
must be held for periods of six months to a year to ensure relatively
complete destruction of Ascaris eggs, for example. Ascaris are
nematode worms that can infest the intestines. They are the most
resistant of the fecal-borne disease parasites in wastes.

Anaerobic
composting may be accomplished in large, well packed stacks or
other composting systems. These should contain 40% to 75% moisture,
into which little oxygen can penetrate, or 80% to 99% moisture
so that the organic material is a suspension in the liquid. When
materials are composted anaerobically, the odor nuisance may be
quite severe. However, if the material is kept submerged in water,
gases dissolve in the water and are usually released slowly into
the atmosphere. If the water is replaced from time to time when
removing some of the material, odor does not become a serious nuisance.

Both
aerobic and anaerobic composting require bacteria. Some bacteria
work better in one or the other environment. Compost piles under
aerobic conditions may attain a temperature of 140° to 160° F in
one to five days depending upon the material and the condition
of the composting operation. This temperature can also be maintained
for several days before further aeration is needed. The heat necessary
to produce and maintain this temperature must come from aerobic
decomposition, which requires oxygen. After a period of time, the
material will become anaerobic unless it is aerated. There is probably
a period between the times when the oxygen is depleted and anaerobic
conditions become evident, during which the process is aerobic.

"Aerobic
composting" requires a considerable amount of oxygen and produces
none of the characteristic features of anaerobic putrefaction.
Aerobic composting can be defined as a process in which, under
suitable environmental conditions, aerobic organisms utilize considerable
amounts of oxygen in decomposing organic matter to fairly stable
humus.

"Anaerobic
composting" describes the process of putrefactive breakdown
of organic matter by reduction in the absence of oxygen where end
products such as CH4 and hydrogen sulfide (H2S) are released.